The present invention relates to a porcelain composition capable of being fired at a low temperature and is best suited for use in a semiconductor element housing package, a wiring board applied to a multi-layer wiring board or the like, to a porcelain and a method of producing the same, and to a wiring board and a method of producing the same. More particularly, the present invention relates to an improvement of a porcelain which can be fired together with a low-resistance conductor material such as copper, silver or gold and has a low dielectric constant, and also is capable of efficiently dissipating heat generated by the operation of an active element such as semiconductor element, for the purpose of reducing signal delay.
Recently, as the information processing technology and communication technology advance in an ever faster rate and are increasingly utilized in everyday life, semiconductor elements are becoming faster in operation and larger in size. As the operation speeds of the semiconductor elements increase, the problem of delay in signal transmission caused by the package, board or the like becomes serious. At the same time, heat resistance of the package and the board poses a significant problem as more heat is generated by the larger semiconductor elements.
In the field of ceramic multi-layer wiring board in the prior art, the most commonly used is an alumina wiring board comprising an insulating layer made of alumina ceramics and a wiring layer made of a high-melting point metal such as tungsten or molybdenum formed on the surface of inside of the insulating layer.
However, in a conventional alumina wiring board, a conductor made of a high-melting point metal such as tungsten (W) or molybdenum (Mo) has a high electrical resistance which, together with the dielectric constant of alumina as high as about 9, results in a significant delay in the signal transmission. For this reason, it has been required to use a low-resistance metal such as copper, silver or gold in place of W or Mo, and decrease the dielectric constant of the insulating layer.
With this background, development efforts have recently been made on wiring boards made of glass ceramics in which the insulating layer is made of glass or glass ceramics which is a composite material of glass and ceramic thereby making it possible to fire the layer at a relatively low temperature of 1050xc2x0 C. or lower so that a low-resistance metal such as copper, silver or gold which has a low melting point can be used as the conductor, and the dielectric constant can be made lower than that of alumina.
For example, Examined Patent Publication (Kokoku) No. 4-12639 discloses a multi-layer wiring board made by firing an insulating layer made of glass with a SiO2 filler added thereto and a wiring layer made of a low-resistance metal such as copper, silver or gold simultaneously at a temperature within a range from 900 to 1000xc2x0 C. Unexamined Patent Publication (Kokai) No. 60-240135 discloses a wiring board made by firing a zinc borosilicate glass with a filler of alumina, zirconia or mullite added thereto together with a metal having a low resistance. Unexamined Patent Publication (Kokai) No. 5-298919 proposes a glass ceramic material wherein mullite or cordierite is precipitated in the crystal phase.
However, the conventional glass ceramic materials such as those described above have a low thermal conductivity within a range from 0.5 to 1.5 W/mxc2x7K, resulting in lower heat dissipation property than alumina and other conventional materials. To solve this problem, Unexamined Patent Publication (Kokai) No. 63-307182 and Unexamined Patent Publication (Kokai) No. 4-254477 disclose wiring boards using glass ceramics made by firing AlN of a high thermal conductivity and glass. Unexamined Patent Publication (Kokai) No. 2-196066 discloses a glass ceramic material made by adding an AlN powder having a small particle diameter and an AlN powder having a large particle diameter of not less 2 xcexcm, forming the mixture into a green body and firing the green body at 950xc2x0 C.
However, when the non-oxide porcelain such as AlN is used as a filler, the glass and the non-oxide ceramic filler react with each other when fired, and the non-oxide porcelain filler is decomposed to evolve a decomposition gas. This gas has caused such problems as swelling of the porcelain, and degradation in the dimensional accuracy, mechanical strength and water absorption. There has also been such a problem that bubbles are generated in the surface of the porcelain, resulting in rough surface and a metallized wiring layer peeling off. Thus it has been difficult to manufacture satisfactory porcelain consistently, resulting in low yield of production and difficulty in volume production thereof as an industrial product. Since such problems become conspicuous when firing in an oxidizing atmosphere such as air, it has been very difficult to form a wiring layer using copper as the conductor, and the fault of removal of the binder is likely to occur when making wiring with copper.
With the method of mixing the glass powder and the AlN powder and firing the green body, it is difficult to make dense glass ceramics at a firing temperatures of 1050xc2x0 C. or lower in case the average particle size of the AlN powder is smaller than 2 xcexcm. This may result in lower mechanical strength of the glass ceramics and/or degradation of the insulation property of the glass ceramics due to water absorption.
Known methods of the AlN powder include the direct nitriding method wherein a metallic Al powder is heated and nitrided at a high temperature of about 2000xc2x0 C. in a pressurized nitrogen atmosphere, and reducing nitriding method wherein an Al2O3 (alumina) powder with carbon added thereto is nitrided in a nitrogen atmosphere. The reducing nitriding method has such problems that the material cost and the production cost are high, and powder having a large particle diameter cannot be made with high purity. The direct nitriding method, on the other hand, requires processing at a high temperature under a high pressure, and, therefore, the metal Al cannot be completely nitrided with a part of the metal Al being left as an impurity. When much metal Al remains in the porcelain, voids are likely to be formed between the glass and the AlN particles. The voids cause such problems as a decrease in the mechanical strength, lower insulation property due to water absorption by the porcelain, higher dielectric constant and an increase of dielectric loss. Supposed cause of the voids being formed between the glass and the AlN particles is the active property of metal Al which results in reaction with the glass powder during firing, thus evolving a gas.
A major object of the present invention is to provide a porcelain composition and a porcelain, which are capable of being fired at a low temperature, together with a low-resistance metal such as copper, silver or gold, and have a high thermal conductivity and a low dielectric constant, and a wiring board using the same.
Another object of the present invention is to provide a porcelain and a method of producing the same having an improved yield of production (i.e. high yield of non-defective units), and a wiring board and a method of producing the same.
The porcelain composition capable of being fired at a low temperature comprises 30 to 95% by weight of a borosilicate glass and 5 to 70% by weight of a non-oxide ceramic filler, wherein said borosilicate glass has a glass transition point of 800xc2x0 C. or lower and a weight loss per unit surface area of said non-oxide ceramics is not more than 0.15 g/cm2 after dipping said non-oxide ceramic having purity of not less than 96% by weight in a glass melt obtained by melting said borosilicate glass with heating at 1200xc2x0 C. for five minutes.
The weight loss described above can be used as an index for evaluation of the reactivity between the borosilicate glass and the non-oxide porcelain filler. When the weight loss is within the range described above, since the reactivity is inhibited, the resulting porcelain composition can be fired together with a low-resistance and low-melting point metal such as copper, silver or gold, particularly silver, and has a high thermal conductivity and a low dielectric constant. Thus the low-temperature fired porcelain and a wiring board using the same can be produced with a high yield.
The low-temperature fired porcelain of the present invention is a dense material comprising a non-oxide porcelain phase and a borosilicate glass phase, and has a relative density of not less than 95% and a thermal conductivity of not less than 2 W/mxc2x7K, and also a ratio of the number of circular voids to the total number of voids is not more than 50% or less as observed in a cross section. The number of circular voids indicates the reactivity between the borosilicate glass and the non-oxide porcelain. The low-temperature fired porcelain having the number of circular voids within the range described above can be preferably used as an insulating board in a wiring board provided with a wiring layer made of a low-resistance metal.
The porcelain is produced by forming the porcelain composition into a predetermined shape, and firing the resulting green body at 1050xc2x0 C. or lower. The firing atmosphere is particularly preferably an oxidizing atmosphere. In the production of the wiring board, a conductive paste made of a low-resistance metal is printed on the surface of the green body, and then fired at 1050xc2x0 C. or lower.
The non-oxide ceramic filler and the non-oxide ceramic phase may comprises at least one selected from the group consisting of AlN, Si3N4, SiC and BN.
The total amount of Pb, Bi, Cu and alkali metal elements in the glass is not more than 0.5% by weight, most effectively, to control the reactivity with the non-oxide ceramic filler. Furthermore, a portion or all of the glass is preferably crystallized by firing the composition to enhance the strength and thermal conductivity.
Another porcelain (ceramic sintered body) of the present invention comprises a spinel compound crystal phase and a non-oxide ceramic phase (non-oxide compound crystal phase) made of at least one selected the group consisting of AlN, Si3N4, SiC and BN, wherein a relative density is not less than 95% and a dielectric constant is not more than 8, and a thermal conductivity is not less than 2 W/mxc2x7K. The porcelain can be fired together with a low-resistance and low-melting point metal such as copper, silver or gold, particularly silver, and can inhibit the reaction between the glass and the non-oxide compound powder, thus making it possible to produce a fired porcelain having a high thermal conductivity and a low dielectric constant with a high yield. Similarly, the porcelain can also be used as an insulating board in a wiring board provided with a wiring layer made of a low-resistance metal.
The spinel compound preferably comprises at least one selected from gahnite (ZnAl2O4) and spinel (MgAl2O4).
In addition to those described above, the sintered body may contain at least one selected from a mullite crystal phase and a cordierite crystal phase as the crystal phase, or preferably contains at least one of a (M1)Al2Si2O8 (M1 is at least one of Ca, Sr and Ba) crystal phase and a (M2)2MgSi2O7 (M2 is at least one of Ca, Sr and Ba) crystal phase.
Such a sintered body is formed by forming a mixed powder containing at least 30 to 95% by weight of a borosilicate glass powder containing at least SiO2, Al2O3, ZnO and B2O3 and 5 to 70% by weight of a non-oxide ceramic filler selected made of at least one selected from the group consisting of AlN, Si3N4, SiC and BN, into a predetermined shape, and firing the resulting green body. The borosilicate glass powder preferably contains 10 to 55% by weight of SiO2, 3 to 35% by weight of Al2O3, 2 to 25% by weight of ZnO, 3 to 25% by weight of B2O3, 0 to 30% by weight of MgO, and 0 to 50% by weight of at least one selected from CaO, SrO and BaO.
The present inventors have studied intensively about properties of the AlN powder in the porcelain obtained by sintering the mixture of the glass powder and the AlN powder. As a result, they have found that a porcelain having a low dielectric constant and a low dielectric loss can be made, while maintaining a high denseness, a high strength, a low water absorption percentage and a high thermal conductivity, by using the AlN powder having a reduced content of metallic Al of hot more than 1000 ppm, which is obtained by treating the AlN powder having an average particle diameter of not less than 2 xcexcm made by the direct nitriding method with a chemical, thereby to dissolve and remove metallic Al remained in the AlN powder. This reason is presumed as follows. That is, the reaction between the glass powder and the AlN powder accompanied with gas evolution is inhibited by reducing the content of metallic Al in the AlN powder, thereby making it possible to densely bond the glass powder with the AlN powder.
That is, still another porcelain (glass ceramics) of the present invention comprises a matrix made of glass and/or a crystal precipitated from the glass and AlN particles having an average particle diameter of not less than 2 xcexcm dispersed in the matrix, wherein the content of metallic Al is not more than 800 ppm based on the total amount.
The total amount of Li, K, Na, Pb and Bi is preferably not more than 15% by weight in terms of oxides (Li2O, K2O, Na2O, PbO and Bi2O3). The amount the AlN particles is preferably within a range from 35 to 80% by weight.
The porcelain can be produced by mixing a glass powder with an AlN powder having an average particle diameter of not less than 2 xcexcm and a metallic Al content of not more than 1000 ppm, forming the mixture, and firing the resulting green body at 1050xc2x0 C. or lower. The glass powder preferably has an average-particle diameter of less than 2 xcexcm.
The wiring board of the present invention comprises an insulating board and a wiring layer formed on the surface of the insulating board and/or inside thereof, wherein the insulating board is made of the porcelain. The wiring layer contains a conductor made of at least one selected from gold, silver and copper.
Various objects as well as other advantages become apparent from the following descriptions.